The present invention relates to a thermal or a thermal transfer printer of the type using a line-type thermal head and, more particularly, to a head drive system associated with such a printer for printing out dots by multi-step modulation. Printers to which the present invention applicable include printers, facsimile terminals and copiers which are implemented with a line-type thermal head.
Generally, a thermal printer or a thermal transfer printer uses a thermal head in which heating elements each being representative of a dot are arranged in one line in a main scanning direction. While a paper is fed by a stepping motor one line at a time in a subscanning direction, the thermal head is driven to generate heat in accordance with data to be recorded so as to print out data line by line. A problem with this type of thermal printer is that when a continuous string of dots are recorded, heat accumulated in the nearby heating elements effect each other to increase the density of dots printed out, resulting in an irregular density distribution as a whole. Although the mutual influence of dots which neighbor each other in the subscanning direction is not so serious because of the feed time of the paper, that of dots which neighbor in the main scanning direction is critical.
In the light of the above, there has been proposed a head drive system which divides the heating elements into an odd dot group and an even dot group and drives the two dot groups on a time division basis, as disclosed in Japanese Patent Application No. 59-217962 by way of example. Although this scheme may successfully preclude the mutual influence of heating elements which neighbor each other in the main scanning direction, it brings about another problem in relation to the representation of halftone. Specifically, when dot modulation is performed to change the size of dots to thereby render halftone, blurring occurs between those dots which have a usual area and those which have a reduced area for rendering halftone and/or those dots which are aligned in an oblique direction become spaced apart from each other. Another drawback inherent in the odd-even dot group scheme is that the energy efficiency is poor in the event of recording a solid picture.
Another drive system for a line-type thermal head which is directed to a uniform density distribution is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 57-34986. The system disclosed is such that when the same heating element is driven continuously, the width of pulses to be applied thereto is reduced and, in addition, a heating element next to a heating element which has just been driven is also applied with a pulse having a reduced width. This scheme, however, is unsuitable for multi-step dot modulation.
Further, there has been proposed a method of automatically adjusting the density of dots printed out. Specifically, the method begins with detecting the level data on a particular heating element to be driven and heating elements located at both sides of that particular heating element. Based on a weight for density compensation which corresponds to the level data, power applied to the particular heating element is controlled in terms of the duration of application or the peak value. This kind of method, however, renders not only an apparatus for practicing the method but also the data processing complicated.